**5.6 Carbon monoxide**

The carbon monoxide is a possible intermediate in the metabolic route of the acetogens and the methanogens (Moletta, 1993); Carbon monoxide was found in a great quantity during toxic inhibition by heavy metals (Liu & al., 2003). According to Moletta (Moletta, 2002) the presence of carbon monoxide is directly related to the acetate concentration, and conversely related to that of methane (Batstone & al., 2002).

**N.B**: there are other process control parameters of the production of biogas during anaerobic digestion, but they do not find any wide application in practice. However, the hydrogen gas is controlled in the gas phase and its measure in the liquid phase enables the identification of the existing different types of bacterial populations which may influence the process of the anaerobic digestion.
